The present invention forms an integral part of a protection system for power transmission lines which are series-compensated with capacitors. Specifically, the invention relates to a method and a device for locating a fault which has occurred on the power transmission line. The distance from a station, located at one end of the power transmission line, to the fault is determined with the aid of calculating algorithms based on impedance models of the power transmission line and voltages and currents measured at the station before and after the occurrence of the fault.
A method for fault location on a three-phase power transmission line is known from U.S. Pat. No. 4,559,491. The method for fault location on a three-phase power transmission line, described in U.S. Pat. No. 4,559,491, is based on an impedance model of the power transmission line. A condition for the method according to U.S. Pat. No. 4,559,491 is that the power transmission line has a linear impedance also when a fault has occurred. Series capacitors for phase compensation are normally provided with overvoltage protection devices in the form of some protection component with a non-linear characteristic in the area of interference and the method according to U.S. Pat. No. 4,559,491 is therefore not applicable to a three-phase power transmission line with series capacitors for phase compensation.
From U.S. Pat. No. 4,719,580 it is known that fault location on a three-phase power transmission line may be made according to method based on a waveguide model of the power transmission line. Fault location on a power transmission line with series capacitors for phase compensation based on a waveguide model of the power transmission line may, for certain types of faults, give a misleading result and this fault-location technique has therefore not been accepted by the users.
A method for fault location based on an impedance model of the power transmission line should take into consideration the fact that the occurrence of a fault on the monitored power transmission line entails the feeding of electric power from both ends of the power transmission line to the site of the fault in question since otherwise the fault location cannot be performed with sufficient accuracy. To obtain a result with satisfactory accuracy in the fault-position determination, fault location based on an impedance model of the power transmission line assumes that the power inputs from the two ends of the power transmission line are determined by measuring or calculating the two power inputs. From U.S. Pat. No. 5,455,776 it is known that fault location on a three-phase transmission line may be made with a method which assumes measurement of voltage and current at the two ends of the monitored transmission line. The method could be adapted to series-compensated transmission lines but has the disadvantage of being dependent on measured values from the two ends of the transmission line, which complicates the fault location and makes it more expensive compared with methods operating with measured values from one end of the transmission line only, normally the end of the transmission line where the fault location is arranged.
A method for fault location based on measurement at one end of the transmission line only is described in IEEE Transaction on Power Apparatus and Systems, Vol. PAS-104, No. 2, February 1985, pp. 424-436. This method assumes that the monitored part of the transmission line is described by a linear model and the method is thus not useful for series-compensated transmission lines with non-linear protection devices for protecting the capacitors against overvoltage.